Surveillance And Analysis Of Four Vertically Transmitted Diseases In Breeding Swine Farms Of China

Porcine reproductive and respiratory syndrome(PRRS),classical swine fever(CSF),porcine circovirus disease(PCVD)and swine pseudorabies(PR)are important vertically-transmitted diseases in commercial swine farms and breeding swine farms in China.These diseases could cause reproductive failure of sows,respiratory problems of piglets and growing pigs,as well as immunosuppressive diseases followed by secondary infections of other diseases,leading to significant economic losses to the swine industry.The "National Medium-and Long-Term Plan for Animal Diseases Prevention and Control(2012-2020)" was launched and implemented in 2012,in which eradication programmes for PRRS,CSF and PR in breeding swine farms are included.In an effort to gain insight into the epidemiology of these four diseases in breeding swine farms in China,in this study an active surveillance was conducted for the detection of PRRSV,CSFV,PCV2,and PRV field virus antibody.Toward this end,samples collected during 2011-2013 from 98 breeding swine farms in 29 provinces/municipalities/autonomous regions were detected by using PCR,real-time PCR,gene sequencing and ELISA.Our results indicated that,from 2011 through 2013,the highest positive rate was observed in PCV2,of which the positive rate of farm was 89.04%-93.15%and the positive rate of individual swine was 22.57%-26.67%.The overall status of PCV2 infection,however,appeared stable;in addition,the positive rate of PCV2 was found significantly higher in tonsil than that in serum(positive rate of individual swine:20.75%vs 2.31%,OR=11.09,95%Cl=7.62-16.13,p<0.001).As for PRRSV,the positive rate of farm was detected to be 89.04%-93.15%and the positive rate of individual animal was 0.96%-1.76%,both were lower than that of PCR2.Likewise,overall,the situation of PRRSV infection remained steady;similarly,the positive mate of PRRSV was found slightly higher in tonsil than that in serum.The positive rate of CSFV was relatively lower,as evidenced by the findings that no CSFV was detected in breeding swine farms from 2011 to 2012.Nevertheless,in 2013 the CSFV-positive rate of farm was 3.23%and the positive rate of individual swine was 0.09%.Lastly,the positive rate of PRV filed virus antibody was unexpectedly high.Interestingly,the proportion of PRV filed virus antibody-positive farm and PRV-positive swine individual increased dramatically in 2012 compared to those in 2011(i.g.positive rate of farm and positive rate of individual swine increased from 36.71%to 64.56%and from 12.78%to 16.6%,respectively).Intriguingly,the proportion of PRV filed virus antibody-positive swine continually increased up to 23.51%in 2013,whereas the proportion of PRV-positive farm decreased dramatically to 39.24%.Our data suggested that PRRSV,PCV2 and PRV were widely distributed in breeding swine farms in China,which may consequently increase the risk of transmission of these viruses from nucleus farms to F1 generation farms,F2 generation farms,parental farms,or even commercial farms.Additionally,it is worth to note that,co-infection of these pathogens afore-mentioned is highly significant,with co-infection of PCV2 and PRV,or PRRSV and PCV2 being the most common and serious.This suggested that great challenge and a variety of weaknesses exist in the imporr of swine,biosafety management,disease prevention and control measures.We suggested improve the national preventative strategies by strengthening the management of approval procedures and disease control measures,and elimination of positive herds for breeding swine farms.The isolated PRRSV and PCV2 strains were then genotyped utilizing real-time PCR and complete genome sequencing.The results showed that two PRRSV genotypes,NA-PRRSV and EU-PRRSV,and two subtypes,NA-PRRSV classic strain and NA-PRRSV mutant strain,have been dominantly prevalent in Chinese breeding farms.A new strain,which showed the highest homology with NADC30 strain that was isolated in the United States in 2008 and classified into NA3,was identified in 2013.In 2011,the positive rate of NA-PRRSV mutant strain was the highest(0.89%),which,however,subsequently decreased slowly to 0.5%;the positive rate of NA-PRRSV classic strain was relatively lower(0.5%),while it increased slowly up to 1.15%;the positive rate of EU-PRRSV was the lowest(0.26%),which decreased eventually to 0.03%in 2013.NA-PRRSV mutant strain was replaced by NA-PRRSV classic strain,resulting in the presence of NA-PRRSV classic strain that appeared to be a dominant epidemic subtype in 2013.We demonstrated that several PCV2 subtypes,including 2a(2D,2E,2F),2b(1 A,1B,1C)and recombinant cluster,were prevalent simultaneously in the breeding swine farms of China.Among them,the highest positive rate was detected in 2b-1C subtype(54.35%-62.5%),suggesting 2b-1C was the dominant genotype.Genotype 2a-2F was found to be less important,since the positive rate was 18.75%一 23.91%.Importantly,our data revealed co-infection of two genotypes or subtypes of PRRSV and PCV2 strains in breeding swine farms,raising the possibility of generation of novel recombinant viruses.Based on our results described above,we underscored the importance of pathogen detection in the process of introducing of new breeding swine,in order to prevent the introduction of exotic virus.Besides,surveillance of co-infection should also be enhanced for the purpose of identifying novel recombinant virus in a rapid manner.Finally,we performed a questionnaire survey.The significant difference in terms of the positive rate of PRRSV,PCV2 and PRV was analyzed between boars and sows,gilts and sows,local breeding swine and imported breeding swine.The results indicated that the positive rate of PRRSV and PCV2 was higher in boars than in sows;it was higher in gilts than in sows;it was also higher in local breeding swine than in imported breeding swine.These data suggested that boars,gilts,and local breeding swine were likely the risky factors of infection of PRRSV and PCV2.Similar analysis was performed in PRV infection,and the results illustrated that sow and imported breeding swine were likely the risky factors of PRV infection.We suggested,therefore,that surveillance should be focused on swine populations with high risk based on the epidemiological characteristics of different diseases.Taken together,our results of systemic epidemiological surveillance and genetic characterization described here helps to trace the group,space,time distribution and phylogenetic evolution of PRRSV,CSFV,PCV2 and PRV in breeding swine farms of China.Our finding provides the basis for implementation of eradication programme of vertically-transmitted diseases in breeding swine farms in China.